Business travelers certainly know by now that their hotel rooms are costing more than they did a few years ago. Less noticed, though, is what's happened to hotel parking rates.

More hotels are charging for parking, and rates are rising sharply at hotels that do.

According to PKF Consulting, hotels that charged for parking over an eight-year period ended in 2006 saw their parking revenue rise 51 percent. Over the same period at those hotels, PKF says, average daily room rates rose 18 percent, and overall revenue rose 14 percent. PKF, an Atlanta-based research company, analyzed data from 383 hotels at the request of USA Today.

Though a higher volume of parked cars accounts for some of the revenue increase, PKF research director Robert Mandelbaum says the higher parking revenues mean guests are paying higher rates.

Many major hotels say they have boosted their rates in recent months, and parking fees can add up to a big slice of a guest's bill.

For example, the daily fee for parking a car at The Peninsula New York, which, like many big-city hotels, offers only valet parking, increased last month from $55 to $60. That's the most expensive rate USA Today found when it called 30 hotels in six cities — New York, Boston, Chicago, Los Angeles, San Francisco and Honolulu — known for high parking rates.

Two Manhattan parking garages within one block of The Peninsula are $16 and $17 cheaper than The Peninsula's daily rate. Of six hotels contacted in Manhattan, The Waldorf-Astoria has the least-expensive rate: $45 for 24 hours.

Outside Manhattan, parking fees are less expensive. In Queens, hotel guests can park their own cars for $22 daily at the New York LaGuardia Airport Marriott or $22 a day at the Wyndham Garden Hotel LaGuardia Airport. "I was blown away by a $45-per-night charge on a family trip to New York a few months ago," says frequent business traveler Warren Kurtzman.

Big-city hotels say the rates are justified. They're providing a service — door-to-door valet parking — that is more convenient for guests and saves them time. Another advantage: Many hotels allow guests to enter and exit parking lots as many times as they wish each day without an extra charge, while public lots charge for each entry.

But business traveler Teresa Colson of Lexington, Ky., says hotels "are charging excessively for self-parking," and parking spaces "are way too small." She often uses valet parking when it's available, because "the price difference is narrowing," and it's safer than self-park garages, she says.

Marc Belsher, who travels frequently to San Francisco, says he parks at hotels less often than in the past. Instead, the healthcare worker from Newberg, Ore., has opted for public transportation.

"I avoid the cost of a rental car, gas, the ridiculous and ever-escalating parking fees, tipping valets, the time lost picking up and dropping off the car and the time lost waiting for the valet to bring the car to the front of the hotel," Belsher says.